Meridian gyros are known, wherein the spin axis of the gyro is kept horizontal, for example by suspending the housing by means of a tape. A gyro directing torque acts on the gyro and tends to align the gyro spin axis with north direction. In order to avoid the loss of time involved with the rotation of the gyro into the north direction, it is known, with such a tape suspended meridian gyro to sense the deflection of the gyro from a zero position by means of a pick-off and to apply the pick-off signal with approximately high gain to a torquer, which exerts on the gyro a torque about the vertical tape axis and counteracting the gyro directing torque. Thus the gyro is electrically restrained to the zero position, and, practically, the gyro directing torque is counteracted by a counter-torque. The amplified pick-off signal, which is applied to the torquer, will then be proportional to the gyro directing torque. The north deviation, i.e. the deviation of the zero position of gyro from the north direction, can be derived from this signal. As the gyro directing torque is proportional to the cosine of geographic latitude, this latitude has to be taken into account when determining the north deviation. Conventionally the gyro spin axis of this meridian gyro is pre-aligned with north (U.S. Pat. No. 3,750,300 to Tumback and U.S. Pat. No. 3,758,952 to Fischel).
Furthermore an electrically restrained gyro with horizontal spin axis is known with which the gyro directional torque is measured at two slightly different azimuth angles of the spin axis. The values obtained thereby are fed into an analog computer which is to compute therefrom the north deviation independently of geographic latitude. With this prior art device, two consecutive measurements with intermediate rotation of the gyro in azimuth through a fixed angle are required. The signal representing north deviation is obtained as a difference of two signals which are large as compared thereto, whereby the accuracy of this measurement is rather limited (U.S. Pat. No. 3,206,864 to Sanchirico).
In another prior art gyro instrument (German Pat. No. 1,281,155) the gyro consists of a rotating ball with well-defined main axis of inertia. The ball is mounted through an air bearing in a cup, which is driven about a vertical axis by a motor. The gyro is driven by the air friction occuring, when the cup is driven, and tends to maintain its orientation in inertial space. However the cup and thus the direction of the driving torque acting on the gyro change their orientation relative to inertial space in the course of the rotation of the earth. Thereby the axis of rotation of the cup will not coincide with the main axis of inertia of the gyro, even if these axes were initially aligned. This results in a precession torque as component of the driving torque acting on the gyro, whereby the main axis of inertia of the gyro is caused to follow the vertical axis of rotation of the cup. The follow-up occurs, however, with a lag in east-west direction, which is observed by means of an optical system.
This prior art arrangement is expensive and makes use of effects, such as torque generation by air friction, which are difficult to control. The position of the gyro has to be detected contact-freely, for example by visual observation or by photoelectric pick-offs.
In another prior art device (German Offenlegungsschrift No. 1,448,737), a spherical gyro is mounted for universal rotation on a central support ball. A force, which is exerted by a torquer, acts on the gyro axis at a distance from the support ball. The torquer is laterally spaced from the gyro and is located on an arm, which is rotatable around the gyro about a vertical axis by means of a servomotor. The position of the gyro axis is picked off in two mutually perpendicular directions by means of photoelectric position pick-offs. The servomotor and the torquer are controlled by the position pick-offs such that the gyro spin axis is kept vertical. The arm will then align itself with north direction, while the erection torque then acting in east-west direction provides a measure of geographic latitude.
Also this arrangement is rather expensive. It provides north direction only after a run-in procedure.
U.S. Pat. No. 4,123,849 discloses a device for determining the north direction by using a two-axis gyro with vertical spin axis. An angle pick-off and a torquer are arranged on each of the two input axes. Each angle pick-off is connected crosswise to the torquer of the respective order input axis through amplifier means, whereby the gyro is electrically restrained to its position of rest. The ratio of the restraining torques is the inverse tangent of the north deviation angle. The gyro is supported on a float assembly which floats on a liquid, whereby the spin axis remains always vertically aligned.
This device requires expensive means for keeping the gyro spin axis vertical.
German Offenlegungsschrift No. 25 45 025 discloses a navigational instrument for the navigation of land vehicles, wherein a north-seeking meridian gyro, for example of the type disclosed in U.S. Pat. No. 3,758,952 or German Offenlegungsschrift No. 1 941 808 is provided for the determination of the north direction with stationary vehicle. A free gyro as heading reference unit is arranged to be aligned in accordance with the meridian gyro. A speed sensor provides a signal proportional to vehicle speed. A computer is connected to the heading reference unit and to the speed sensor and provides output signals representing the vehicle position in a grid coordinate system, from the grid heading angle and speed signals supplied by the heading reference unit and the speed sensor. The drift of the free gyro relative to the grid coordinate system due to the rotation of the earth is compensated for or is taken into account in the computer.
It is an object of the invention to provide a navigational instrument for vehicles, wherein a single gyro can be used both for the "northing" with stationary vehicle and as heading reference unit during the mission.
It is another object of the invention to provide a navigational instrument with a gyro for northing, wherein no exact alignment of the gyro spin axis is required.
It is another object of the invention to eliminate certain errors occurring with the "northing".
It is a still further object of the invention to provide a heading-attitude reference unit for a navigational instrument, which unit provides the heading and the transformation parameters for a transformation from a vehicle-fixed coordinate system into an earth-fixed coordinate system.
A more specific object of the invention is to obtain heading and transformation parameters unaffected by Newton's accelerations of the vehicle relative to ground.
Eventually it is an object of the invention to provide estimated values of the errors of some measured quantities used, and to correct the measured quantities correspondingly.
According to one aspect the invention relates to a navigational instrument for a vehicle, wherein the north direction is determined by means of a gyro, comprising: a two-axis gyro having a spin axis, a first and a second input axis, a first angle pick-off and a first torquer on the first input axis, a second angle pick-off and a second torquer on the second input axis, first amplifier means for applying the amplified angle signal from the first angle pick-off to the second torquer, second amplifier means for applying the amplified angle signal from the second angle pick-off to the first torquer, and signal processing means, to which the amplified angle signals are applied. This navigational instrument is characterized by the following features:
The gyro with the angle pick-offs and the torquers is arranged in an intermediate housing. The intermediate housing is mounted for rotation about an axis of rotation parallel to one input axis through 90.degree. from a first position with substantially vertical spin axis into a second position. A pair of vehicle-fixed accelerometers is arranged with its input axes parallel to the transverse and longitudinal axes, respectively, of the vehicle. The signal processing means comprise first computer means for providing initial vehicle attitude signals from the amplified angle signals with stationary vehicle and said first position of the intermediate housing, and second computer means for continuously providing vehicle attitude signals representing the attitude of the moving vehicle in an earth-fixed coordinate system from said initial vehicle attitude signals and said accelerometer signals with the second position of the intermediate housing.
Said first computer means may comprise means for forming from the acceleration signals A.sub.x.sup.F, A.sub.y.sup.F of the accelerometers estimated values of the elements C.sub.31 and C.sub.32 of the directional cosine matrix for the transformation from a vehicle-fixed coordinate System (x.sup.F,y.sup.F,z.sup.F) into an earth-fixed coordinate system (x.sup.R,y.sup.R,z.sup.R) in accordance with the relation ##EQU1## means for forming, from the estimated values thus obtained, an estimated value of the third element C.sub.33 of the last line of the directional cosine matrix C.sub.F.sup.R in accordance with the relation. ##EQU2## means for providing, from the signals C.sub.31,C.sub.32 and C.sub.33 as well as from signals representing the rotary speeds W.sub.y.sup.F and W.sub.x.sup.F, which are derived from the signals applied to the torquers, a signal representing the initial heading angle .psi.(O) of the vehicle in an earth-fixed coordinate system in accordance with the relation ##EQU3## wherein .PHI. is geographic latitude and .omega..sub.E is the rotary speed of the earth.
Certain systematic errors may be eliminated by measuring at two positions of the gyro unit angularly offset by 180.degree. about a horizontal input axis and/or about the vertical spin axis.
The attitude parameter and the heading angle during the mission can be obtained in that the signal processing or computer means comprises means for providing signals EQU C.sub.31 =C.sub.32 .omega..sub.z.sup.F -C.sub.33 .omega..sub.y.sup.F EQU C.sub.32 =C.sub.33 .omega..sub.x.sup.F -C.sub.31 .omega..sub.z.sup.F wherein
C.sub.31,C.sub.32,C.sub.33 are the elements of the last line of the directional cosine matrix, PA1 C.sub.31,C.sub.32 are the associated time derivatives, PA1 .omega..sub.x.sup.F is the rotary speed about an input axis x.sup.F in the vehicle-fixed coordinate system, PA1 .omega..sub.y.sup.F is the rotary speed about the second input axis y.sup.F in the vehicle-fixed coordinate system, and PA1 .omega..sub.z.sup.F is the rotary speed about the third input axis z.sup.F in the vehicle-fixed coordinate system, PA1 C.sub.32,C.sub.33 are elements, provided by the transformation parameter computer, from the last line of the directional cosine matrix for the transformation from a vehicle-fixed coordinate system into an earth-fixed coordinate system, PA1 d.sub.z,d.sub.y are the known drifts of the rotary speed sensors sensitive about the vertical and transverse axes, respectively, and PA1 .DELTA.C.sub.31 is an estimated value of the error of the element C.sub.31 of the directional cosine matrix.
means for integrating the signals C.sub.31 and C.sub.32 with respect to time to provide signals C.sub.31 and C.sub.32, respectively, means for providing a signal ##EQU4## from the signals C.sub.31 and C.sub.32 thus obtained, means for feeding the signals C.sub.31,C.sub.32 and C.sub.33 back to the computer for providing C.sub.31 and C.sub.32 from the rotary speed signals, means for providing a signal from the signals C.sub.31,C.sub.32,C.sub.33 thus obtained and from the rotary speed signals W.sub.z.sup.F and W.sub.y.sup.F, and means for integrating this signal with respect to time to provide a signal representing the heading angle .psi. in the earth -fixed coordinate system.
According to another aspect, the invention relates to a navigational instrument for land vehicles. The navigational instrument includes an inertial measuring unit having rotation-responsive inertial sensors, which respond to rotary movements about vehicle-fixed axes, and accelerometers, which respond to linear accelerations along vehicle-fixed axes. A speed sensor responds to the speed of the vehicle with respect to ground in the direction of the longitudinal axis of the vehicle. A transformation parameter computer to which the signals from the inertial measuring unit are applied comprises means for computing transformation parameters for the transformation of vector components from a vehicle-fixed coordinate system into an earth-fixed coordinate system. Corrective signal generators, to which transformation parameters from the transformation parameter computer are applied, provide output signals representing the components due to gravity of the accelerations detected by the accelerometers. The output signals are superposed to the signals from the accelerometers to provide translation acceleration signals. Integrators, to which the translation acceleration signals are applied, provide inertial speed signals. An optimal filter, to which the inertial speed signals and the signal from the speed sensor are applied provides optimized speed signals referenced to vehicle-fixed coordinates on the basis of these signals. A coordinate transformation computer to which the optimized speed signals and the transformation parameters from the transformation parameter computer are supplied comprises means for transforming these speed signals into transformed speed signals, which are referenced to an earth-fixed coordinate system. A position computer to which the transformed speed signals are supplied comprises means for providing position signals representing the position of the vehicle.
Therein the optimal filter has the following structure: The difference (V.sub.IX.sup.F -v.sub.x.sup.F) of the component v.sub.Ix.sup.F falling into the direction of the vehicle longitudinal axis of the inertial speed signal and of the speed signal from the speed sensor is opposed, at a first summing point, to the difference of a first signal and a second signal to provide a first difference signal (z.sub.1), said signal representing an estimated value (.DELTA.v.sub.Ix.sup.F) of the error of the longitudinal component signal (v.sub.Ix.sup.F) of the inertial speed, and said second signal representing an estimated value (.DELTA.v.sub.x.sup.F) of the error of the speed signal (v.sub.x.sup.F). The component (v.sub.Iy.sup.F) falling into the direction of the vehicle transverse axis of the inertial speed signal is opposed, at a second summing point, to a further signal to provide a second difference signal (z.sub.2), this further signal representing an estimated value (.DELTA.v.sub.Iy.sup.F) of the error of the transverse component signal (v.sub.Iy.sup.F) of the inertial speed. The first difference signal (z.sub.1) multiplied by a factor (K.sub.11) and a signal EQU C.sub.32 d.sub.z -C.sub.33 d.sub.y
are added at a third summing point, the sum being integrated by means of an integrator to provide a signal .DELTA.C.sub.31, wherein
The signal C.sub.31 multiplied by the acceleration g due to gravity, the first difference signal z.sub.1 multiplied by a factor K.sub.31, as well as the known zero deviation b.sub.x of the accelerometer sensitive in longitudinal direction of the vehicle are added at a fifth summing point and are integrated by means of an integrator to provide a signal which represents the estimated value .DELTA.v.sub.Ix.sup.F of the error of the longitudinal component signal v.sub.Ix.sup.F of the inertial speed. The signal A.sub.x.sup.F of the accelerometer sensitive in longitudinal direction of the vehicle, and the element C.sub.31 of the first column and third line of the directional cosine matrix provided by the transformation parameter computer and multiplied by the acceleration g due to gravity are added at a sixth summing point to provide a signal v.sub.x.sup.F representing the translatory acceleration in the longitudinal axis of the vehicle. The first difference signal z.sub.1 multiplied by a factor K.sub.61 is integrated by an integrator to provide an estimated value of the error .DELTA.k.sub.x of the scale factor of the speed sensor. The first difference signal z.sub.1 multiplied by a factor K.sub.51, and the product of the signal representing the translatory acceleration v.sub.x.sup.F and of the output signal from the last-mentioned integrator are added at a seventh summing point, the sum being integrated by means of a further integrator to provide the signal which represents the estimated value .DELTA.v.sub.x.sup.F of the error of the speed signal from the speed sensor. The signal C.sub.32 multiplied by the acceleration g due to gravity, and the second difference signal z.sub.2 multiplied by a factor K.sub.42, as well as the known zero deviation of the accelerometer sensitive in longitudinal direction of the vehicle are added in an eighth summing point and are integrated by means of an integrator to provide a signal which represents the estimated value (.DELTA.v.sub.Iy.sup.F) of the error of the transverse component signal v.sub.Iy.sup.F of the inertial speed. The signal representing the estimated value .DELTA.v.sub.x.sup.F of the error of the speed signal is substracted from the speed signal.
An embodiment of the invention will now be described in greater detail with reference to the accompanying drawings: